1. Field of the Invention
The present invention relates to a method for transmitting information symbols between a first transceiver and a second transceiver by modulating a carrier signal.
2. Description of the Related Technology
The types of information are designated by the term information symbol. Examples of types of information, in addition to the actual digital data words, are control signals such as clock information. In previous methods of contactless communication, the types of information are coded and digitally transmitted in the form of data words by means of modulating a carrier wave. An important area of application for this is the area of contactless identification, in which particularly transponder technology is used. These systems consist of a transponder, which is frequently integrated into a keyring pendant or a so-called smart card, and a stationary base unit which, in the case of applications in the automotive field, is fitted in an automobile. The communication between transponder and base unit is based upon an inductive coupling, in which the data is transmitted by means of a modulated carrier wave. Particularly in applications in the automotive field, the complete authentication between base station and transponder must be completed within a period of some 100 ms so that the user does not notice any delay. The information must be transmitted between transponder and base station at a high data rate in order to implement the ever higher security requirements for the authentication process. This requirement is difficult to fulfill, especially in so-called passive systems in which the transponder does not have its own power supply, and has to obtain all its energy from the electromagnetic field of the base station by means of absorption modulation. To do this, the transponder has to be located in the electromagnetic near-field region of the base station in order to be able to absorb enough energy because the inductive coupling is low in the medium air. As the near-field region is substantially larger at low frequencies, the carrier frequencies lie in the range around 20 MHz in passive systems, which enables a range of a few cm to be attained.
Methods which function according to the previous state of the art are described, for example, for passive transponder systems in the data book of TEMIC Semiconductor GmbH, 2000, p. 319 and p. 333. In this method, all types of information are coded in the form of data words, and modulated in digitized form upon a carrier frequency. In order to decode this information in the data words, a system clock, which is gained from the carrier frequency by using dividing stages, is required in the transponder system.
The disadvantage of previous methods is that, in order to be able to decode the information contained in the data words in the receiving station, additional digital data (overhead) has to be generated by coding all types of information in the form of data words based on a protocol. This lowers the data transmission rate correspondingly. This results in additional waiting time for decoding the transmitted information, which is disturbing in time-critical applications. Another disadvantage of the previous method is that, in passive systems for decoding the transmitted data words, the system clock is derived from the frequency of the carrier with the aid of a dividing stage. With a frequency in the 20 MHz range, the dividing stage has to switch frequently within the transponder in order to generate the system clock and, in combination with the further switchover for clock generation, requires a considerable proportion of the energy which is gained by absorption modulation from the field of the base station. This substantially reduces the communication distance between base unit and transponder.